Method for the protection against corrosion of a steel part made of austentic or semi-austentic steel during the production of sulfuric acid

ABSTRACT

This invention relates to a method for the protection against corrosion of steel parts made of austenitic or semi-austenitic steel during the production of sulfuric acid. To improve the corrosion resistance of the steel parts which are in contact with the sulfuric acid, it is proposed to use austenitic or semi-austenitic steel which has a Cr content of 15 wt-% to 36 wt-% and an Ni content of 9 wt-% to 60 wt-% and in which the ratio of the chemical elements (Cr+Si)/(Ni+Mo) lies in the range from 0.9 to 1.9 or in which the ratio of the chemical elements Cr/(Ni+Mo) lies in the range from 0.8 to 1.5, and to additionally provide this steel part with an anodic corrosion protection.

[0001] This invention relates to a method for the protection against corrosion of steel parts made of austenitic or semi-austenitic steel during the production of sulfuric acid.

[0002] In general, sulfuric acid is produced by the catalytic conversion of the SO₂ content of gases to obtain SO₃ and—in the case of dry gases—by the subsequent absorption of the SO₃ formed in concentrated sulfuric acid or—in the case of humid gases—by the subsequent condensation of the sulfuric acid formed. The usual technical components such as drier, absorber, heat exchanger etc. get in contact with concentrated sulfuric acid starting at about 93 wt-% and an elevated temperature. This sulfuric acid is extremely aggressive and exerts a fast and strong corrosion on the structural parts to be used. Therefore, the structural parts which get in contact with sulfuric acid must be made of corrosion-resistant materials. Special ferritic steel alloys, cast iron, plastics, ceramics, glass or other materials with a corresponding lining turned out to be particularly corrosion-resistant. For these applications, the non-metallic materials generally have unfavorable mechanical properties and are difficult to process. The metallic materials have good mechanical properties, but their corrosion resistance is not sufficient, or the materials are very expensive. In particular for use in heat exchangers, very thin-walled components are required, which need a high transfer of heat. In these components, the previous corrosion resistance no longer is sufficient. These plants are usually operated with a sulfuric acid concentration≧93 wt-% to 100 wt-% and a temperature up to 140° C. A known method of corrosion protection is the anodic corrosion protection. In this method, the materials to be protected are coated with a metal oxide layer which prevents the corrosion attack. exchangers, very thin-walled components are required, which need a high transfer of heat. In these components, the previous corrosion resistance no longer is sufficient. These plants are usually operated with a sulfuric acid concentration≧93 wt-% to 100 wt-% and a temperature up to 140° C. A known method of corrosion protection is the anodic corrosion protection. In this method, the materials to be protected are coated with a metal oxide layer which prevents the corrosion attack.

[0003] The use of austenitic steels during the production of sulfuric acid is known from EP 0 130 967. The steel grades indicated in this protective right are intended in particular for use in heat exchangers. The materials used here do not satisfy the requirements which must now be fulfilled by corrosion-resistant materials. For the technical plants now in use smaller corrosion rates are required in particular.

[0004] DE 38 30 365 describes the use of ferritic chromium-molybdenum steels which are resistant to sulfuric acid with a concentration from 94 wt-% onwards and with a temperature below the boiling point. These ferritic steels are very expensive and more difficult to process than austenitic steels. The corrosion resistance is not regarded as sufficient either.

[0005] Proceeding from this prior art, it is the object underlying the invention to improve the protection against corrosion of at least one steel part of a device made of austenitic or semi-austenitic steel during the production of sulfuric acid, in which device the steel part gets in contact with the sulfuric acid.

[0006] In accordance with the invention, this object is solved in the above-mentioned method in that at a sulfuric acid concentration of 93 wt-% up to 100 wt-% and a temperature of 140° C. up to the boiling point of the sulfuric acid, the steel part is made of austenitic or semi-austenitic steel which has a Cr content of 15 wt-% to 31 wt-% and an Ni content of 9 wt-% to 60 wt-%, and in which the ratio of the chemical elements (Cr+Si)/(Ni+Mo) lies in the range from 0.9 to 1.25, and in which the steel part has an anodic corrosion protection, wherein an anode, a cathode and a reference electrode are connected with a potentiostat which supplies an adjustable direct electric current, and wherein the cathode and the reference electrode are in contact with the sulfuric acid and the anode is in contact with the steel part.

[0007] Experiments have shown that steel grades with a Cr content of 15 wt-% to 36 wt-% and an Ni content of 9 wt-% to 60 wt-% are particularly resistant to corrosion.

[0008] In terms of corrosion resistance, especially the elements silicon and chromium from among the chemical alloying elements are known to form a passive layer, whereas the chemical elements nickel and molybdenum weaken the formation of a passive layer.

[0009] The ratio of the chemical elements (Cr+Si)/(Ni+Mo) in the range from 1.01 to 1.25 turned out to be particularly advantageous.

[0010] Likewise, for those steel grades which only have a minor content of silicon there was obtained an advantageous ratio of the chemical elements Cr/(Ni+Mo) in the range from 0.8 to 1.5, preferably from 0.8 to 1.1.

[0011] The ratio is particularly favorable when molybdenum is present in a not too large amount of 0 wt-% to 2.5 wt-%. Depending on the availability of the steel grades to be supplied for the raw materials such as tubes or sheets, austenitic or semi-austenitic steel parts with a molybdenum content of 2 wt-% to 2.5 wt-% can be used.

[0012] What is particularly critical for corrosion are those ranges in which the concentration of sulfuric acid is about 97 wt-% to 99 wt-% or the temperature of sulfuric acid is about 1 60° C. to 230° C.

[0013] During the production of sulfuric acid, components particularly susceptible to corrosion are heat exchangers, such as e.g. plate-type heat exchangers or shell-and-tube heat exchangers, as well as the entire pipe system.

[0014] Embodiments of the process will be explained by way of example with reference to the drawing, in which:

[0015]FIG. 1 shows the current density/potential curve of an austenitic material,

[0016]FIG. 2 is a schematic representation of the anodic protection in a heat exchanger.

[0017] Embodiments of the process will be explained by way of example with reference to the drawing, in which:

[0018]FIG. 1 shows the current density/potential curve of an austenitic material,

[0019]FIG. 2 is a schematic representation of the anodic protection in a heat ex-changer.

[0020]FIG. 1 shows the current density/potential curve of a typical austenitic material containing 16.5 to 18.5 wt-% chromium, 11 to 14 wt-% nickel and 2 to 2.5 wt-% molybdenum. In this measurement, sulfuric acid was used as medium with 98 wt-% at a temperature of 200° C. As cathode, there was used a steel cathode made of 1.4404. On the abscissa, the potential is plotted in millivolt (mV) against a Hg/HgSO₄ reference electrode, and on the ordinate the current density is plotted in milliampere per square centimeter (mA/cm²). There can also be used other reference electrodes, such as e.g. a calomel electrode or a cadmium bar.

[0021] The first part of the diagram in the range from 0 to 600 mV shows a peak which is referred to as active potential. In the range from 600 mV to 1800 mV then follows the saddle of the curve, the so-called passive potential. The subsequent rise from 1800 mV is referred to as transpassive potential. To achieve a corrosion protection as effective as possible in the anodic corrosion protection, the current density must lie within the range of the passive potential. The values represented here are exemplary, as they are material- and temperature-dependent.

[0022]FIG. 2 shows the arrangement of the anodic corrosion protection in a shell-and-tube heat exchanger (1) for sulfuric acid. Via a connection (2), cooling medium is introduced into a first chamber (3) of a shell-and-tube heat exchanger (1). From there, the cooling medium is distributed and flows through individual tubes (4) into a second chamber (5), from which the cooling medium is discharged again. By way of example, only two tubes (4) are represented here.

[0023] Via a further connection (6), hot sulfuric acid (2) is introduced. The sulfuric acid flows around the tubes (4) filled with cooling medium and is discharged again via the connection (7). When flowing around the tube bundles (4), the sulfuric acid is cooled.

[0024] A plurality of metal cathodes (8) are mounted between the tubes (4) in the shell-and-tube heat exchanger. The representation shows a cathode (8) by way of example. The number of cathodes (8) used depends on the size of the heat ex-changer and also on the temperature and the concentration of the sulfuric acid. The cathode (8) is made of the material 1.4404 and is in permanent contact with the sulfuric acid. The cathode (8) is connected with the negative pole of a potentiostat (9) by an electric line. The potentiostat (9) is a d.c. voltage source whose positive pole (10) is connected with the parts of the shell-and-tube heat ex-changer (1) to be protected via an electric line.

[0025] A second reference electrode (11) is inserted in the shell-and-tube heat ex-changer via a seal and is connected with the potentiostat (9) via an electric line. This reference electrode (11) likewise is permanently surrounded by the sulfuric acid and provides the measurement basis for the potentiostat (9). By means of the electric voltage between reference electrode (11) and cathode (8), the potential required for the corrosion protection is determined and adjusted at the potentiostat (9).

[0026] In the subsequent Table, the corrosion behavior of the materials in accordance with the invention is shown at different temperatures and a sulfuric acid concentration of 98 wt-%. The flow rate of the sulfuric acid was 1 m/s. The corrosion behavior was determined by immersion tests. In all cases, the test period was 7 days. The removal rates were determined by measuring the corrosion flow and by conversion to mm/a. The test medium was renewed after each test cycle. Temperature [° C.] 1.4571 1.4404 1.4465 1.4591 Corrosion 160 0.02 0.03 0.15 — rate 180 0.06 0.04 0.06 0.01 mm/a 200 0.04 0.10 0.14 0.11

[0027] Thus, the corrosion rates are distinctly lower than in the previous prior art. 

1. A method for the protection against corrosion of at least one steel part of a device which is used in a plant for producing sulfuric acid, and in which the steel part contacts concentrated sulfuric acid, comprising at a sulfuric acid concentration of 93 wt-% up to 100 wt-% and a temperature of 140° C. up to the boiling point of the sulfuric acid, using as the steel part is one made of austenitic or semi-austenitic steel which has a Cr content of 15 wt-to 31 wt-% and an Ni content of 9 wt-% to 60 wt-%, and in which the ratio of the chemical elements (Cr+Si)/(Ni+Mo) lies in the range from 0.9 to 1.25, and in which the steel part has an anodic corrosion protection, connecting an anode, a cathode and a reference electrode with a potentiostat which supplies an adjustable direct electric current, and placing the cathode and the reference electrode in contact with the sulfuric acid and the anode in contact with the steel part:
 2. The method as claimed in claim 1, wherein the ratio of the chemical elements (Cr+Si)/(Ni+Mo) lies in the range from 1.01 to 1.25.
 3. The method as claimed in claim 1, wherein the ratio of the chemical elements Cr/(Ni+Mo) lies in the range from 0.8 to 1.5.
 4. The method as claimed in claim 3, wherein the ratio of the chemical elements Cr/(Ni+Mo) lies in the range from 0.8 to 1.1.
 5. The method as claimed in claim 1, wherein the steel part is made of austenitic or semi-austenitic steel which has a molybdenum content of 0 wt-% to 2.5 wt-%.
 6. The method as claimed in claim 1, wherein the steel part is made of austenitic or semi-austenitic steel which has a molybdenum content of 2 wt-% to 2.5 wt-%.
 7. The method as claimed in claim 1, wherein the concentration of the sulfuric acid lies in the range from 97 wt-% to 99 wt-%.
 8. The method as claim 1; wherein the temperature of the sulfuric acid is about 160° C. to 230° C.
 9. The method as claimed in claim 1, wherein the steel part is used in a heat exchanger.
 10. The method as claimed in claim 1, wherein the steel part is used in an acid-conducting pipe. 